Magnetic lenses

ABSTRACT

Apparatus in which a beam of charged particles is brought to a focus by means of a magnetic lens situated behind the target position, leaving the area in front of the target clear, the magnetic lens comprising an electrically conducting coil arranged around the axis of the beam and a magnetic polepiece which extends along the axis of the beam at least within the space surrounded by the coil. Preferably the polepiece protrudes and at least partly tapers towards the target position.

1 1 Mar. 11, 1975 1 1 MAGNETIC LENSES [75] lnventor: Thomas Mulvey,Birmingham,

England [73] Assignee: The National Research Development Corporation,London, England 22 Filed: Aug. 31, 1973 21 Appl. No.: 393,532

[301 Foreign Application Priority Data Sept, 4, 1972 Great Britain40888/72 [52] U.S. C1. 250/398, 250/396 [51] Int. Cl. H0lj 37/26 [58]Field of Search 250/396, 398; 313/89 56] References Cited UNITED STATESPATENTS 2,438,971 4/1948 Hiller 250/398 3,560,739 2/1971 Wolff 250/3983,587,013 6/1971 Dietrich et a1 250/396 3,629,578 12/1971 Le Poole250/398 3,687,716 8/1972 Steigerwald 250/398 Primary Examiner-Archie R.Borchelt Assistant E.\'aminerB. C. Anderson Attorney, Agent, orFirm-Elliott l. Pollock [57] ABSTRACT Apparatus in which a beam ofcharged particles is brought to a focus by means of a magnetic lenssituated behind the target position, leaving the area in front of thetarget clear, the magnetic lens comprising an electrically conductingcoil arranged around the axis of the beam and a magnetic polepiece whichextends along the axis of the beam at least within the space surroundedby the coil. Preferably the polepiece protrudes and at least partlytapers towards the target position.

9 Claims, 3 Drawing Figures MAGNETIC LENSES This invention relates tomagnetic lenses for the focusing of beams of charged particles, .inparticular beams of electrons.

It is well known to focus a beam of electrons by causing it to passaxially through a magnetic field of symmetrical distribution, themagnetic field being produced by a current carrying coil positionedaround the beam. The focusing of electron beams is required, forexample, in electron beam cutting or welding apparatus, in theilluminating system of electron microscopes of the transmission type, inscanning electron microscopes and in electron probe X-ray microanalysisapparatus. In all these forms of apparatus, but particularly in the lastnamed form of micro-analysis apparatus, an electron beam is required tobe focused on a small target area. It is desirable that the area inwhich the beam is concentrated should be very small and compact, and itis therefore essential that the aberrations of the lens should be assmall as possible.

Conventional magnetic lenses which are used in these-forms of apparatusare positioned about the path of the beam and may accupy a considerabledistance along the beam. Such lenses so positioned can imposeundesirable structural and design limitations on the apparatus in whichthey are used. Thus, for example, in electron microscopes, magneticcondenser lens systems are employed which are closely positioned aroundvthe electron beam so that a very powerful pumping system is required tomaintain the desired vacuum conditions within the constrictedpassageways.

X-ray micro-analysis depends on the detection of X- rays emitted fromthe region of a specimen bombarded by electrons, the X-rays beingcharacteristic of elements present at the region of bombardment. The X-rays are emitted in all directions relative to the surface of thespecimen, from normal almost to grazing angle. It has been acharacteristic of previously known electron probe X-ray micro-analysisapparatus that the X-ray emission has been intercepted over anappreciable solid angle by the lens structure through which the electronbeam hashad, to pass to be focused on the specimen. This has caused lowsensitivity of detection of the emitted X-rays, and mechanicaldifficulties in the placing of devices for viewing the specimen and ofX-ray spectrometers for analysing the emitted X-rays into their variouscomponents.

A magnetic lens system, as described in British Pat. Specification No.1,253,652 having a conducting wire coil positioned behind the targetposition with respect to the beam of particles, was invented to overcomethese mechanical difficulties. A magnetic lens system has now beendevised which in its optimal configuration provides a substantialimprovement over the aforementioned magnetic lens system.

According to the invention, an apparatus comprises a source arranged toproduce a beam of charged particles and a magnetic lens which providessubstantially the only means for focusing the said beam onto a targetposition between the said source and magnetic lens, the magnetic lenscomprising an electrically conducting wire coil arranged around saidaxis and a polepiece composed of material of high magnetic permeabilitywhich extends along said axis within the space surrounded by the coil.The term electrically conducting wire coil includes a single coil or aplurality of coils electrically connected so that, when suitableexcited, they will produce the magnetic field required to focus the beamonto the aforesaid target position.

Preferably the coil is circular and the magnetic polepiece is ofcircular cross-section. The polepiece may lie partially or wholly withinthe space surrounded by the coil and in the majority of applications itwill occupy at least half this space. The cross-sectional area of thepolepiece is preferably as large as the dimensions of the coil willallow and optionally at least part of the polepiece tapers in thedirection of the source of charged particles. The total angle of taperis preferably greater than in a number of preferred applications, thepolepiece may have a snout which projects beyond the side of thewire'coil facing the source of charged particles. The snout may beshaped symmetrically about the axis of the beam of charged particles soas to induce a magnetic field configuration required for focusing thebeam of charged particles in a desired manner. Such a magnetic fieldconfiguration may for example demand a high localized flux density andit will be recognized by a person skilled in the art that care must beexercised to ensure that the magnetic material of the polepiece is notunnecessarily driven into saturation in such a situation. If it is foundto be necessary, the polepiece may be tapered or stepped along itslength to overcome problems of saturation. This may be required, forexample, if the snout projects a considerable distance beyond the sideof the coil facing the source of charged particles.

In a given application, it is advantageous to reduce the reluctance ofthe magnetic circuit linking the coil as much as possible withoutaltering the essential focusing properties of the coil. Thus it is usualto minimize the length of the magnetic circuit in air or in vacuo byconfining the magnetic circuit to a layer of magnetic material for thatpart of its length where it does not contribute to the focusing of thebeam of charged particles provided both that the magnetic field requiredfor focusing the beam can be achieved with the given polepiececonfiguration and that the given polepiece is mechanically suitable forthe given application.

According to a preferred feature of the invention therefore, themagnetic lens has a layer of material of high magnetic permeabilityarranged on the side of the coil remote from the source of chargedparticles. The layer of magnetic material is preferably continuouswithin the periphery of the coil and is integral with or is in intimatecontact with the central cylindrical polepiece.

The layer of magnetic material may also extend around the periphery ofthe coil and in some circumstances this peripheral magnetic layer mayproject beyond the side of the coil facing the source of chargedparticles. In a further embodiment, the aforesaid layer of magneticmaterial may extend to cover an outer portion, but not the whole, of theside of the coil facing the source of charged particles. The peripheral,or extended, magnetic layer may be suitably shaped so as to modify themagnetic field distribution.

In some applications, an apparatus according to the invention iscontained within a single vacuum chamber. In such applications, themagnetic lens is preferably constructed to have vacuum propertiescompatible with the conditions required within the chamber. This mayconveniently be done by encapsulating the electrically conducting wirecoil in a vacuum-tight enclosure, which enclosure may be formed byeither a part or the whole of the combination of the aforementionedlayer of magnetic material and the magnetic polepiece together with alayer of non-magnetic material (for example brass or non-magneticstainless steel) joined together with vacuum-tight seals to complete thesaid enclosure. However, it is an important feature of the inventionthat the magnetic lens may be situated outside the vacuum chamber behindthe target position, thereby simplifying the construction of themagnetic lens which is no longer required to be compatible with thevacuum conditions existing within the chamber.

Preferably the elctrically conducting wire coil of the magnetic lens ismade as small as possible consistent with the satisfactory operation ofthe invention. This is not essential, particular when in a preferredapplication the magnetic lens is so constructed that the magneticcircuit is confined by a layer of magnetic material for the greaterportion of its length in such a case, the magnetic-field will beapproximately uninfluenced by the geometry of the coil and the magneticfield strength on the axis of the lens will depend chiefly on theproduct of the number of turns of the coil and the electric currentpassing through them. However it is normally advantageous to minimizethe size of the coil as this enables the total size of the magnetic lensto be kept to a minimum, thereby reducing the possibility of setting upstray magnetic fields which might affect the electron optical propertiesof the system.

The electrically conducting wire coil may be wound either with wire ofany convenient cross-sectional shape or with thin metal strip. Anodizedaluminum tape may conveniently be employed as in this form ofconstruction there is no need for insulation over and above thatprovided by the oxide layer since the voltage drop from one turn to thenext is relatively small. The coil may be in the form ofa thick disc orplate asdescribed in British Pat. Specification No. 1,253,652 as thesuperior cooling properties of such a coil allow higher electriccurrents to be used. The size of the magnetic lens may be significantlyreduced by comparison with conventional lenses by providing the coilwith cooling means located, for example, within the layer of magneticmaterial or within the structure of the coil, thereby allowing arequired magnetic field to be generated by a larger electric currentflowing in a smaller number of turns of wire.

In a preferred application, in which the wire coil is encapsulated in awater tight enclosure, water is circulated around the wire of the coiland the electrical connections to the coil may conveniently be made viainsulated wires which are taken out of the enclosure via the water inletand outlet pipes. This method of construction may be preferable when theenclosure is also vacuum tight and the coil is contained in a vacuumchamber together with the remainder of the apparatus.

According to an optional feature of the invention, the magneticpolepiece of the lens may be provided with a small circular axial holethroughout its length to permit the passage of any portion of the beamof charged particles which has been transmitted through the target ontowhich it has been focused by magnetic lens. The hole may be of uniformdiameter along its length or it may he stepped or tapered to allow forany spreading in the beam of transmitted charged particles. The holewill normally be of a diameter small enough to cause little or nodisturbance of the magnetic field used to focus the beam. In a preferredconfiguration in which the magnetic lens has a layer of magneticmaterial applied to the side of the coil remote from the source ofcharged particles and the layer is continuous within the periphery ofthe coil, the aforementioned hole is extended through this layer alongthe axis of the lens. A magnetic lens provided with an axial hole may beused as a barrier between two volumes within a vacuum chamber containingan apparatus in accordance with the invention. The dimensions of thehole may be such that the hole has a low conductance to air at roomtemperature under conditions of molecular flow and thus it may bepossible to maintain a pressure differential across the hole.

An important practical advantage of the invention is that a magneticlens, as herein described may be conveniently fitted into an electronbeam or like apparatus so as to leave the whole space between the sourceof charged particles and the target position around the beam of chargedparticles available for siting a radiation detector or the like.

A further practical advantage of the invention is that a magnetic lensas herein described, in focusing a beam of charged particles, canreadily introduce a lower level of spherical aberration into theelectron optical system of the apparatus than can be achieved usingconventional lenses. This is because the aberration dependspredominantly upon the dispositioning of the polepiece which can beaccurately machined and positioned, rather than on the coil windingwhich requires great care to be evenly wound. However in someapplications requiring a very high level of symmetry of the magneticfield, it may be useful to use sheets of conducting material havingradial symmetry to provide electrical connections to the wire coil, asdescribed in U.S.. Pat. specification No. 1,253,652.

In addition, a magnetic lens as herein described provides an essentiallyradial magnetic field to focus a beam of charged particles. This may beparticularly useful as it allows, for example, secondary electronsdisplaced from the target position to be collected at a point about theaxis of the beam close to the lens without the need for a relativelylarge electrostatic field of attraction to overcome the effect of thefocusing magnetic field on the secondary electrons.

The invention will be further described by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a diagrammatic sectional view of apparatus showing therelative position of a magnetic lens, a target and a beam of chargedparticles and FIGS. 2 and 3 illustrate in section along their axis twoexamples of magnetic lenses incorporating a number of the preferredfeatures of the invention.

Referring to FIG. 1, 11 represents diagrammatically a source of chargedparticles, hereinafter referred to for convenience as electrons,collimated by means not illustrated into a beam 12. Concentric with theaxis of the beam 12, produced, and in a plane normal thereto, issituated a coil 13 of. annular shape, having a front face 14 facing thesource 11 and a back face 15. A cylindrical magnetic polepiece 16,manufactured from eg soft iron or other magnetic material of highpermeability, occupies the whole of the volume surrounded by the coil13. Polepiece 16 projects from the coil 13, a short distance towards thesource 11 and is provided with a flange'16A, also of magnetic material,which backs the coil 13. The magnetic lens system defined by the coil 13and the magnetic material 16 and 16A can be suitably excited to causethe beam 12 of electrons to be brought to a focus at a target position Fsituated at a point on the axis of the beam 12 between the front face 14of the coil and the source 11. A specimen 17 may be placed so that Flies in its surface. The specimen may be microwelded, or investigated,for example, as the object of electron microscopical examination or byelectron probe X-ray micro-analysis. It is for the latter purpose thatthe present invention is particularly advantageous, since there is verylittle restriction of the angle over which the emitted X-rays may bedetected, whereas, as explained above, there is considerable restriction with magnetic lenses of known form. An X-ray detecting device18 is shown in block form, receiving radiation. along a directionindicated by the broken line 19. Also shown as an alternative is acollecting device 20, of annular form, which may be positioned to becoaxial with the beam 12 and held at a positive potential with respectto the specimen 17 to collect secondary electrons produced byinteraction of the beam 12 with the specimen 17.

A more sophisticated example of the magnetic lens as herein desscribedis illustrated in FIG. 2. This lens is constructed to have a circularcoil 13, which is a combination of two annular coils 21 and 22, and acylindrical magnetic polepiece 16 which has a snout 23 which issymmetrical about the axis of the polepiece l6 and which projects beyondthe front face 14 of the coil 13 towards the source 11. Integral withthe polepiece 16 is a layer 24 of material of high magnetic permeabilitywhich is applied to the back of the coil 13 and which extends around theperiphery of the coil 13. The lens is positioned to be coaxial with thebeam 12 and can be suitably excited to cause the beam 12 of electrons tobe brought to a focus at a target position F situated at a point on theaxis of the beam 12 between the front face 14 of the coil 13 and thesource 11.

The polepiece 16 and the layer of magnetic material 24 are provided witha small hole 25 drilled along the axis of the lens to allow for thepassage of any part 26 of the beam 12 which may be transmitted through aspecimen 17 placed so that F lies in its surface. That part 26 of thebeam 12 may be collected by a suitable detector 27.

The lens is constructed to allow for water cooling of the coil 13. Aplate 28 of non-magnetic material (for example non-magnetic stainlesssteel) 'is bolted (by means not shown) to the layer of magnetic material24 and together with the layer of magnetic material 24 and the polepiece16 forms an enclosure for the coil 13. O- ring seals 29 and 30 areprovided as shown to make the enclosure water-tight and two non-magneticstainless steel pipes 31 and 32 are argon-arc welded into the layer ofmagnetic material 24 to allow for the inflow and outflow of water tocool the coil 13.

The electrical connections to the coils 21 and 22 are made via insulatedwires (not shown) which are brought out of the lens enclosure via thewater pipes 31 and 32. The coils 21 and 22 may be electrically connectedto power supplies (not shown) in any way suitable to produce themagnetic field required to focus the beam 12 at the target position F.

A further example of a magnetic lens as herein described is illustratedin FIG. 3. This lens is constructed basically the same as the lens shownin FIG. 2, having a coil 33 which is a combination of two annular coils34 and 35. The lens however has a radially symmetric polepiece 36 whichtapers along the whole of its length in the direction of the source ofcharged particles 11 and which projects beyond the face 37 of the coil35 towards the source 11.

The polepiece 36 tapers at a total angle of 45along the larger part 38of its length and at a total angle of along the smaller part 39 of itslength which projects beyond the magnetic lens towards the source 11.The coil 35 is wound to have a smaller inner diameter than its companioncoil 34 in order to make better use of the available space.

The polepiece 36 is provided with a circular hole 40 along its axisthroughout its length to allow for the passage of any part 26 of thebeam 12 which may be transmitted through a specimen 17 placed so that Flies in its surface. The diameter of the hole 40 increases by stepsalong its length away from the source 11 of charged particles to allowfor spreading of the part 26 of the beam 12 transmitted through thespecimen 17.

We claim:

1. An apparatus which comprises a source operative to produce a beam ofcharged particles along a given axis, and a magnetic lens which providessubstantially the only means for focusing the said beam onto a targetposition on the axis between the said source and magnetic lens, the saidmagnetic lens being operative to provide a substantially radial field inthe focusing region, said lens being located on the side of said targetposition opposite to said source and comprising an electricallyconducting wire coil having a front face which lies in a planeperpendicular to the said given axis, the axis of said coil beingcoaxial with said given axis, said lens further comprising a polepiececomposed of material of high magnetic permeability which extends alongsaid core axis towards said source at least within the space surroundedby the coil.

2. An apparatus according to claim 1 wherein said polepiece occupies atleast half of the space surrounded by the coil.

3. An apparatus according to claim 1 wherein said polepiece projectsbeyond the said front face of the electrically conducting wire coiltoward the source of charged particles. 1

4. An apparatus according to claim 1 wherein at least part of saidpolepiece tapers in the direction of the source of charged particles.

5. An apparatus according to claim 4 in which the total angle of taperis not less than 45.

6. An apparatus according to claim 1 in which the magnetic lens has alayer of material of high magnetic permeability arranged on that side ofthe coil which is opposite to said front face and remote from the sourceof charged particles, the reluctance between said layer and saidpolepiece being low.

7. An apparatus according to claim 6 in which said layer of material ofhigh magnetic permeability extends around the periphery of the coil andpresents to the source of charged particles an end face having radialsymmetry.

8. An apparatus according to claim 7 in which said electricallyconducting wire coil is encapsulated in a fluid-tight enclosure which isformed by the combination of said layer of material of high magneticpermeability and said polepiece sealed to a layer of nonmagneticmaterial to complete the saidenclosure, and

means for circulating fluid through said enclosure throughout itslength.

1. An apparatus which comprises a source operative to produce a beam ofcharged particles along a given axis, and a magnetic lens which providessubstantially the only means for focusing the said beam onto a targetposition on the axis between the said source and magnetic lens, the saidmagnetic lens being operative to provide a substantially radial field inthe focusing region, said lens being located on the side of said targetposition opposite to said source and comprising an electricallyconducting wire coil having a front face which lies in a planeperpendicular to the said given axis, the axis of said coil beingcoaxial with said given axis, said lens further comprising a polepiececomposed of material of high magnetic permeability which extends alongsaid core axis towards said source at least within the space surroundedby the coil.
 1. An apparatus which comprises a source operative toproduce a beam of charged particles along a given axis, and a magneticlens which provides substantially the only means for focusing the saidbeam onto a target position on the axis between the said source andmagnetic lens, the said magnetic lens being operative to provide asubstantially radial field in the focusing region, said lens beinglocated on the side of said target position opposite to said source andcomprising an electrically conducting wire coil having a front facewhich lies in a plane perpendicular to the said given axis, the axis ofsaid coil being coaxial with said given axis, said lens furthercomprising a polepiece composed of material of high magneticpermeability which extends along said core axis towards said source atleast within the space surrounded by the coil.
 2. An apparatus accordingto claim 1 wherein said polepiece occupies at least half of the spacesurrounded by the coil.
 3. An apparatus according to claim 1 whereinsaid polepiece projects beyond the said front face of the electricallyconducting wire coil toward the source of charged particles.
 4. Anapparatus according to claim 1 wherein at least part of said polepiecetapers in the direction of the source of charged particles.
 5. Anapparatus according to claim 4 in which the total angle of taper is notless than 45*.
 6. An apparatus according to claim 1 in which themagnetic lens has a layer of material of high magnetic permeabilityarranged on that side of the coil which is opposite to said front faceand remote from the source of charged particles, the reluctance betweensaid layer and said polepiece being low.
 7. An apparatus according toclaim 6 in which said layer of material of high magnetic permeabilityextends around the periphery of the coil and presents to the source ofcharged particles an end face having radial symmetry.
 8. An apparatusaccording to claim 7 in which said electrically conducting wire coil isencapsulated in a fluid-tight enclosure which is formed by thecombination of said layer of material of high magnetic permeability andsaid polepiece sealed to a layer of nonmagnetic material to complete thesaid enclosure, and means for circulating fluid through said enclosurearound the coil in said enclosure to cool said coil.